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pattern will be taken. Beam convergence angles generally
range between 2 x 10"3 rads to 20 x 10"3 rads. The
resulting pattern consists of a number of diffracted
discs, each disc corresponding to a diffracted beam.
The selected area diffraction pattern is formed using
a large beam that is essentially parallel. The area from
which the diffraction pattern is taken is defined by an
aperture, the selected area aperture. Beam convergence is
usually on the order of 1 x 10"^ rads. The resulting
pattern consists of diffraction spots. Each spot
corresponds to a diffracted beam.
The convergent beam electron diffraction (CBED)
pattern is usually formed in the back focal plane of the
objective lens, just as is the diffraction pattern in the
selected area diffraction mode (Steeds, 1979). The CBED
pattern contains a wealth of information about the
crystallography of the diffracting crystal, in many cases
much more information than is contained in the selected
area diffraction pattern. This information appears in the
discs of the pattern and can be used in 1) identification
of the diffracting crystal's point and space groups
(Buxton et al., 1976), 2) identification of Burgers
vectors (Carpenter and Spence, 1982), 3) the measurement
of local lattice parameters (Jones et al., 1977), 4) the
measurement of foil thickness (Kelly et al., 1975),